The thermally functioning actuator consists of a pressure cylinder comprising a tube (10), two end closure parts (11, 12) each fixed to one end of this tube, a body of liquefiable material housed in this tube between the said closure elements, and a piston (15) passing through a passage made in the closure part (12) and extending into the liquefiable material. During assembly, the body of liquefiable material is arranged in the tube between the closure elements and bearing against the latter, while at least one of the said closure elements is not fixed to the tube, so that the body of liquefiable material determines the distance separating the closure elements, the latter being fixed to the tube by folding down the peripheral edge (21, 22) of the ends of the latter inwards.
